1. Convection over the Northern South China Sea during the Onset of the East Asian Summer Monsoon
Richard H. Johnson Colorado State University
Acknowledgments: Paul Ciesielski, Mike Toy, Andy Newman, Tom Keenan, Steve Aves
Symposium on Ocean and Atmosphere Research in the South China Sea Taipei, January 2016
4 June 2008

2. Cool water develops along China coastline during winter monsoon
(Johnson and Houze 1987)

3. 11 January 2016

4. JUNE-JULY
Summer monsoon onset over northern SCS in mid-May: Northeasterlies switch to southwesterlies; warm air flow over cool water
(Johnson and Houze 1987)

5. SOP-mean QuikSCAT winds: warm air flowing over cool water
MAY-JUNE 2008
SOP-mean QuikSCAT winds: warm air flowing over cool water

6. Flow Across Meandering SST Front
Stable conditions prevent vertical mixing of higher momentum from aloft
Adapted from Chelton and Xie (2010)

7. SOP-mean QuikSCAT winds: warm air flowing over cool water
MAY-JUNE 2008
SOP-mean QuikSCAT winds: warm air flowing over cool water

9. Control and Smoothed SST WRF-ARW Runs
SMTHSST adjusted to match GFS SST values at boundaries
SST gradient in SMTHSST not unlike some years (later)
13-18 JUNE
2008
(Toy and Johnson 2014, JAS)

10. Rainfall Enhancement Due to SST Gradient
 Difference in rainfall between smoothed and 2008 observed SST gradient around Taiwan
 Parts of southwest Taiwan receive ~30% (125 mm) more rainfall over five-day period (13-18 June)
* Taipei
* Hualien
* Tainan
* Kaohsiung
Toy and Johnson (2014, JAS)

11. South China Sea Monsoon Experiment (SCSMEX) – May-June 1998
Study of key processes associated with onset and evolution of SE Asia summer monsoon
BMRC C-POL RADAR

12. Mesoscale Convective System (MCS) Organization
(based on 88 linear, warm-sector MCSs, May 1996,97)
(58%)
6-8 h
(19%)
4-6 h
(19%)
4-6 h
(Parker and Johnson 2000)

13. South China Sea Monsoon Experiment (SCSMEX) – May-June 1998
Study of key processes associated with onset and evolution of SE Asia summer monsoon
BMRC C-POL RADAR

14. SCSMEX 6-hour Radar Animation
5 June 1998 TS ●
Hong Kong
BMRC C-POL RADAR

15. SCSMEX 4-hour Radar Animation
18 May 1998 LS ●
Hong Kong

16. SCSMEX 6-hour Radar Animation
8 June 1998 PS ●
Hong Kong

17. STUDY PERIOD
May-June 1998

18. CONVECTION ON OUTFLOW BOUNDARIES
SCSMEX Modes of Organization as a Function of the Low and Midlevel Wind Shears
Low-level shear layer: hPa
Mid-level shear layer: hPa
(>4 m s-1)
SHEAR- BAND
CONVECTION ON OUTFLOW BOUNDARIES
SHEAR- BAND
SHEAR- BAND
SHEAR- BAND
Classification based on LeMone et al. (1998)
(>5 m s-1)
SHEAR- BANDS
(Johnson et al. 2005, MWR)

19. Heavy rainfall in coastal regions
Ten-year ( ) May-June Mean Rainfall
(Johnson 2011)
Heavy rainfall in coastal regions
TRMM 3B42

20. Heaviest rainfall occurs along shore – Western Ghats and Myanmar: suggests topography and diurnal cycle likely important (sea/land, mountain/valley breezes)

21. Normalized Amplitude, Mean Diurnal Cycle of May-June Rainfall (1998-2007)
(Johnson 2011)
Morning maxima: base of Himalayas, lee of Tibetan Plateau, interior Borneo
Afternoon/evening rainfall maxima over enclosed ocean basins
Morning rainfall maxima along coastlines

22. Time of Maximum May-June Rainfall (1998-2007)
Diurnally migrating or propagating convective features
~15 m s-1
~10-15 m s-1
~10 m s-1
~13 m s-1
~10-15 m s-1
Morning rainfall maxima

23. Morning precipitation, moves southward
May-June 1998
(Aves and Johnson 2008)
Morning precipitation, moves southward

24. N S N S Evening LLJ LLJ Morning Sea breeze South China Sea
Some convection may move inland in the early morning and dissipate (Chan and Ng 1993)
LLJ
Land breeze or other effects? N S
South China Sea
Morning
(Johnson 2006)

25. Summary
SST pattern at summer monsoon onset leads to low-level convergence, precipitation over northern SCS; affects Mei-yu rainfall over Taiwan
Three organizational modes of MCSs observed during 1998 SCSMEX: TS, LS, and PS
Organization of convective systems determined by low and midlevel wind shear
Prominent diurnal cycle of convection over northern SCS during summer monsoon onset characterized by southward propagation of MCSs from mainland China

26. Summer Monsoon Onset Dates (Tao and Chen 1987)

27. Diurnal Oscillations of 850-hPa Wind (May-June 1998)V 08 6 02
Hong Kong 08
LLJ: Inertial Oscillation 14 4 02 14 20
Dongsha Island 20 2
|v|850 at 0200 L at Hong Kong 2 4 6
U (m s-1)